The present invention relates, in general to a vertical cavity surface emitting laser (VCSEL).
A VCSEL is a semiconductor laser diode in which the laser oscillation and the optical emission occur in a direction normal to the pn junction plane. This device has many properties which are advantageous when compared to the more commonly used edge-emitting laser diodes. These properties include: low optical beam divergence, a circular optical output, and single longitudinal mode operation. In addition, VCSEL devices are manufactured in a manner which allows wafer level testing of individual devices. These properties make VCSELs attractive in applications such as optical data storage, data communication and laser scanners.
In most systems utilizing semiconductor lasers, dynamic stabilization and control of the optical output power is required. In edge emitting laser diodes this is usually accomplished by allowing a portion of the light emitted by the laser diode, i.e., using light emitted from the laser diode's rear mirror for bias control, to illuminate an optical photodetector which produces a photocurrent proportional to the intensity of the light emitted by the laser diode. This optically generated photocurrent, in turn drives a feedback circuit which controls the laser bias current. However, due to the structure of the VCSEL, light emitted from the rear mirror is absorbed by the GaAs substrate. Thus, the system of using light emitted from the rear mirror is unavailable for bias control.
Monolithic integration of the photodetector to detect laser light from either the top or bottom mirrors has been attempted, "Monolithic Integration of Photodetector With Vertical Cavity Surface Emitting Laser", G. Hasnain et al., Electronic Letters, Vol. 27, No. 18, Aug. 29, 1991, pp. 1630-1632. This implementation, while attractive, is complex and may be very costly to manufacture. Integration of a lateral detector has also been proposed, "Feedback Mechanism for Vertical Cavity Surface Emitting Lasers", End Tabatataie, U.S. Pat. No. 5,285,466. This technique has the limitation of only detecting the emission of spontaneous light from the laser. This results in a photocurrent which is not proportional to the total optical power of the VCSEL.
An optical package has been devised for efficiently providing automatic optical feedback for an optical emitter. However, the present optical packages include a metal header and can (TO-46 or 56 cans) as shown in FIG. 1, numeral 100. FIG. 2, numeral 200, illustrates steps for the manufacturing process for a metal can VCSEL package s is known in the art. Typically the header (201) is manufactured (202) with, for example, three leads (203). The present optical packages are typically assembled by die bonding (208) a VCSEL (110; 206) and a photodetector (Photo Diode, 104; 204) onto the header (201). In a final assembly, the metal can/cap, with a hologram (108) adhered to a glass window (Cover Glass, 102), is thermally welded (214) to the header (112).
However, this process has a relatively high cost due to the stringent tolerance/alignment requirements needed for the hologram to reflect a laser beam precisely onto the photodetector (104). The hologram (108) must be attached to a glass window (102). A metal can/cap is manufactured (210), and the glass-hologram assembly must then be attached (212) to the lid of the metal can (TO Can Lid, 106). The metal generally gives rise to parasitic inductor-capacitor coupling to the wirebond (i.e., to the VCSEL and photodetector pads) and/or the header, thus limiting the high speed operation of the VCSEL package to less than 1.5 Gigabits/sec.
Since the issues of cost and speed of operation are critical to the performance and manufacture of the VCSEL package, there is a need for a low cost fabrication device, method, bar code scanner and optical storage unit for providing optical feedback for an optical emitter using holograms wherein the interference due to parasitic inductor-capacitor coupling is minimized.